This invention relates generally to a process of forming a sintered article of powder metal by using graphite, silicon carbide and pre-alloyed iron base powder and particularly relates to a method and apparatus of spheroidizing following sintering by forced gas cooling from approximately 1000xc2x0 C. by fan cooling.
Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered article.
Typically the percentage of carbon steel lies in the range of up to 0.8% C. Ultra high carbon steels generally speaking have carbon contents between 0.8% to 2.0% carbon.
It is known that tensile ductility decreases with an increase in carbon content and accordingly ultra high carbon steels have historically been considered too brittle to be widely utilized. However, the strengthening effect of carbon in steels is well understood.
Ultra high carbon steels have been produced as disclosed in U.S. Pat. No. 3,951,697 as well as in the article by D. R. Lesver, CKSYNA. Goldberg, J. Wadsworth and OD SHERBY, entitled xe2x80x9cThe Case for Ultra High Carbon Steels As Structural Materialsxe2x80x9d appearing in the Journal of Minerals, Metals and Materials St., August 1993.
Generally speaking the brittleness of such high carbon steels results from carbides which precipitate during the austentite to ferrite transformation during cooling. Moreover the reference to spheroidization refers to any thermo mechanical process that produces a rounded or globular form of carbide. Spheroidization is the process of heat treatment that changes embrittling grain boundary carbide and other angular carbides into rounded or globular form. In the prior art, the spheroidization process was time consuming and uneconomical as the carbides transform to a rounded form only very slowly. Typically spherodization requires long soak times of several hours at temperature. Mechanical working at elevated temperature has been used to speed up the spherodization process. However this adds costs and is only possible for relatively simple shapes.
The applicant herein has improved the prior art process of producing sintered metal articles having relatively high densities that include heat treatment steps which rapidly spherodize embrittling carbides. For example, applicant obtained U.S. Pat. No. 5,516,483 which relates to a process of forming a sintered article of powder metal comprising blending carbon and ferro alloys, lubricant with iron powder then high temperature sintering the article in a reducing atmosphere then spherodizing the sintered ultra high carbon steel. The use of silicon is disclosed but added as a ferro alloy namely ferro silicon to the iron powder.
Moreover applicant has obtained U.S. Pat. No. 5,552,109 which relates to a high density sintered alloy and spheroidization method utilizing pre-alloyed powders with for example 0.85% Mo in the pre-alloyed form blended with graphite and lubricant. U.S. Pat. No. 5,552,109 exhibits excellent results utilizing a spheroidization method where cooling may occur by oil quenching.
It is an object of this invention to provide an improved powder metal method whereby high density products are produced by spheroidizing with the rapid cooling utilizing a fan in a reducing or neutral atmosphere.
Although U.S. Pat. No. 5,516,483 taught the use of silicon such silicon was added in the form of ferro alloy namely ferro silicon. Generally speaking graphitization elements such as nickel and silicon (other than as trace elements) are to be avoided as taught in U.S. Pat. No. 5,541,922. Moreover if silicon is added as elemental silicon it tends to oxidize which is detrimental to the sintered powder metal article in both fatigue or endurance properties.
Silicon has been added to copper based sintering as shown in U.S. Pat. No. 2,372,203 as well as for cutting tools as shown in U.S. Pat. No. 4,011,108 and also in aluminium alloys as taught by U.S. Pat. No. 4,711,823.
It is a further object of this invention to provide simplified apparatus and method for producing sintered powder metal articles.
An aspect of this invention relates to a process of forming a sintered article of powder metal comprising blending graphite, Si carbide, and lubricant with pre-alloyed iron based powder; pressing said blended mixture to a shaped article; sintering said article in a reduced atmosphere; force gas cooling said sintered article. Silicon inhibits the formation of coarse blocky carbides and therefore permits a slower cooling rate to be utilized, which in turn results in less part distortion and simplified part handling during processing.
Another aspect of the invention relates to a process of sintering articles of powder metal comprising blending graphite, Si carbide and lubricant with pre-alloyed iron base powder; pressing said blended mixture to a shaped article; preheating said pressed article to a temperature between 600xc2x0 C. and 700xc2x0 C.; sintering said article in a furnace in a reducing atmosphere to a temperature between 1250xc2x0 C. and 13500xc2x0 C.; transferring said sintered article from said furnace to a region at a temperature of approximately 980xc2x0 C.; rapidly forced gas cooling said sintered article from 980xc2x0 C. to approximately 300xc2x0 C. to 400xc2x0 C. in nitrogen and further cooling to room temperature; reheating said article in a furnace to approximately 850xc2x0 C. and holding the temperature of approximately 850xc2x0 C. for up to two hours; slow cooling said article to room temperature. In one aspect the pressed article is placed on a tray for the preheating, sintering, transferring, rapidly forced gas cooling, cooling to room temperature steps referred to above, and then the article is separated from the tray prior to reheating said article in said furnace.
It is another aspect of this invention to provide an apparatus for producing sintered articles of powder metal comprising means for blending a mixture of graphite, Si carbide, lubricant and pre-alloyed iron base powder; means for compacting said blended mixture to a shaped article; means for preheating said shaped article to a temperature between 600xc2x0 C. and 700xc2x0 C.; a furnace for sintering said preheated article at a sintering temperature between 1250xc2x0 C. and 1350xc2x0 C. in a reducing atmosphere; means for transferring said sintered article to a transfer zone at approximately 980xc2x0 C.; forced gas means for rapidly cooling said sintered article to approximately 300xc2x0 C.; means to cool to room temperature; means to reheat said article to approximately 850xc2x0 C. so as to slowly cool said article to room temperature.
Another aspect of this invention relates to a process of sintering articles of powder metal comprising blending graphite, Si Carbide and lubricant with pre-alloyed iron base powder, pressing said blended mixture to a shaped article; preheating said pressed article to a temperature between 600xc2x0 C. and 700xc2x0 C.; sintering said article in a furnace in a reducing atmosphere to a temperature between 1250xc2x0 C. and 1350xc2x0 C.; transferring said sintered article from said furnace to a region to slow cool said article to room temperature; reheating said article in another furnace to approximately 980xc2x0 C. and holding the temperature of approximately 980xc2x0 C. for up to one hour; rapidly forced gas cooling said sintered article from 980xc2x0 C. to approximately 300xc2x0 C. to 400xc2x0 C. in nitrogen; then reheating said article in said other furnace to approximately 850xc2x0 C. and holding the temperature of approximately 850xc2x0 C. for up to two hours; and slow cooling said article to room temperature. In yet another aspect the pressed article is placed on a tray for the preheating, sintering, cooling to room temperature steps referred to above; and then the article is separated from the tray prior to reheating said article in said other furnace.
Alternatively, the sintering furnace and other furnace can be linked with automated tray removal means being used.
Another aspect of this invention relates to apparatus for producing sintered articles of powder metal comprising means for blending a mixture of graphite, Si Carbide, lubricant and pre-alloyed iron base powder; means for compacting said blended mixture to a shaped article; means for preheating said shaped article to a temperature between 600xc2x0 C. and 700xc2x0 C.; a furnace for sintering said preheated article at a sintering temperature between 1250xc2x0 C. and 1350xc2x0 C. in a reducing atmosphere; means for transferring said sintered article to a region to cool said article to room temperature; means for reheating said article in another furnace to approximately 980xc2x0 C.; forced gas means for rapidly cooling said sintered article to approximately 300xc2x0 C. to 400xc2x0 C.; means for reheating said sintered article to approximately 850xc2x0 C. so as to slowly cool said article to room temperature.